Hello!
As before on other forums too, this becomes a competition of beliefs.
Found couple of pictures of German naval smoke generators (search terms were "smoke generator" and "kriegsmarine"):
http://www.collectrussia.com/sBoot/smoke.jpg
http://www.collectrussia.com/sboot/smoke1_small.jpg
The pics are from site:
"Schnellboot Weapons and Tactics"
http://www.prinzeugen.com/Weapons.htm
It seems to be a simple pressure bottle thingie. Easily fitted into/under a Bf 109. I suppose wieght wouldn't be restriction too. The only thing I don't understand is the need for the "pole" underneath.
The pole is quite likely a wing strut adapted to this use (whatever it is). Beign streamlined it decreases drag considerably when hanging vertically under the plane. Tube from the smoke generator bottle would be easy to lead through the "pole".
About the other guesses:
The pole/tube is a bit too big and robust for the purpose of altitude sensing or message picking. That kind of hooks could be seen under late 20's-30's reconnaissance planes. The ones I've seen have all been simple rod things and (relatively) snug against fuselage bottom plus usually on centerline.
Mausi had a 150 kW generator run by diesel motor. Where would be the room in Bf 109? IMO there is no point of testing any radio euiqpment in single seater fighter. No operator/testing personnell, no room, not much electical power, limited carrying capability etc.
Upto now we only have one response which has answer from someone being there at the time - and it says smoke generator. It is also the only thing against I can't make any serious argument.
Cheers,
Kari